Submarine fiber optic communication systems carry a large majority of the information that is transmitted between the world's continents. These fiber optic communication systems remain in-place on the bottom of the ocean under thousands of feet, and even miles, of water for many years. Due to the difficulties encountered when having to repair, replace, or generally service these systems, it is desirable that these systems be highly reliable.
Submarine fiber optic communication systems typically include repeaters that appear at regular intervals along the spans of undersea cables to amplify the optical signals traversing the constituent fibers. Other assemblies that may be found along a submarine communication system including branching units, which allow multiple cable stations to be served from a single cable. To protect the sensitive components and/or connections that are housed within these submerged assemblies, a rugged hermetically sealed structure must be employed.
Typically, the optical fibers found within optical repeaters are circular in cross-section, and are constructed of glass surrounded by a protective jacket that is thicker than the glass. For example, a typical glass fiber, which may be referred to as a "glass fiber", "bare fiber", or "unjacketed fiber", can have an outer diameter of approximately 0.010 inches, and a typical jacketed fiber can have an outer diameter of approximately 0.040 to 0.060 inches.
The glass fiber is relatively fragile. Because even microscopic damage to the glass fiber can adversely affect the reliability of the optical repeater (and, as a result, the reliability of the entire submarine optical fiber cable system), care is normally taken to protect the glass fiber from damage. Generally, the likelihood of damage to the glass fiber can be reduced by ensuring that any curvature in the glass fiber meets or exceeds the minimum bending radius of the glass fiber. However, the minimum bending radius of the glass fiber is a function of the expected life of the glass fiber. For example, when at least a 25-year life is expected, the glass fiber typically has a minimum bending radius of approximately 1 inch. This is referred to as the reliability-adjusted minimum bending radius of the glass fiber, because meeting or exceeding this value provides acceptable reliability from bending damage during the expected life of the glass fiber.
Typically, the optical components found within optical repeaters are manufactured with a segment of optical fiber attached at each end and cut to a specified length. Each fiber segment contains a jacketed portion of specified length located adjacent to the optical component, and a bare portion of specified length extending from the opposite end of the jacketed portion. The bare portion is spliced into the bare portion of another segment in the repeater's optical circuit. Creating these splices can be a complicated task, requiring substantial lengths of bare fiber on each side of the splice. Optimally however, the repeater or branching station is designed to be as space-efficient as possible, thereby minimizing its production, storage, shipping, and installation costs. Thus, it is desirable to store each optical fiber segment in the most space-efficient manner possible.
Typically, this involves storing the fiber in a coiled configuration on a tray upon which are mounted at least some of the optical components served by that fiber. Typical trays include a well that extends partially through the thickness of the tray, and an elongated circular spool surrounded by the well. A gap between the spool and the well defines a fiber storage space within which the coiled fibers can be stored.
Delivering a fiber to the fiber storage space typically involves resting an elongated annular mandrel upon the top of the spool, and winding the fiber around the mandrel. Then, the fiber is urged down the mandrel and into the storage space. Both the mandrel and the spool typically have outer diameters that at least meet the reliability-adjusted minimum bending radius of the fiber.
There are numerous disadvantages to the known mandrel. For example, because the mandrel is typically only supported by its bearing down upon the top of the spool, the known mandrel is likely to tip over when a fiber is being wound therearound. This tipping can cause the mandrel to fall upon one or more fibers or optical components, potentially causing damage to a fiber or component that may not be apparent until after the optical repeater or branching station has been placed in service.
Also, the known mandrel is dimensioned to position the wound fiber over the fiber storage space, but very close to the spool. This positioning can cause the fiber to bunch-up around the spool when urged off the mandrel. Bunching of the fiber can cause the fiber to inhabit substantially more of the storage area's vertical dimension than is necessary, thus preventing the maximum number of fibers from being stored in the storage area.